FMA Unit 3.3 A Timer and Counter

A Timer and Counter

The microcontroller 8051 has two 16 bit Timer/Counter register namely Timer 0 (T0) and Timer (T1). Both these register can be configured independently as a timer or Counter.

Timer:

·         If the timer incremented by Internal Clock then it is called as Timer.

·         It is a specialized type of clock which is used to measure time intervals.

·         A timer that counts from zero upwards for measuring time elapsed is often called a stopwatch.

·         It is a device that counts down from a specified time interval and used to generate a time delay.

·         The register incremented for every machine cycle i.e 12 oscillator periods.

·         Maximum count rate is 1/12^th  of the oscillator frequency.

Counter:

·         If the timer incremented by External Clock then it is called as Counter.

·         A counter is a device that stores (and sometimes displays) the number of times a particular event or process occurred, with respect to a clock signal.

·         It is used to count the events happening outside the microcontroller.

·         The register is incremented in response to high to low of the corresponding to an external input pin (T0, T1). Hence it requires two machine cycles i.e 24 oscillator periods.

·         Maximum count rate is 1/24^th  of the oscillator frequency.

Timers of 8051 and their Associated Registers

The 8051 has two timers, Timer 0 and Timer 1. They can be used as timers or as event counters. Both Timer 0 and Timer 1 are 16-bit wide. Since the 8051 follows an 8-bit architecture, each 16 bit is accessed as two separate registers of low-byte and high-byte.

Timer 0 Register

·         The 16-bit register of Timer 0 is accessed as low-byte and high-byte.

·         The low-byte register is called TL0 (Timer 0 low byte) and the high-byte register is called TH0 (Timer 0 high byte).

·         These registers can be accessed like any other register.

·         For example, the instruction MOV TL0, #4H moves the value into the low-byte of Timer #0.

Timer 1 Register

·         The 16-bit register of Timer 1 is accessed as low- and high-byte.

·         The low-byte register is called TL1 (Timer 1 low byte) and the high-byte register is called TH1 (Timer 1 high byte).

·         These registers can be accessed like any other register.

·         For example, the instruction MOV TL1, #4H moves the value into the low-byte of Timer 1.

FMA Unit 3.4 Timer Registers

To control timer operation two special Functions Register are used

1.         TCON (Timer Control Register)

2.         TMOD (Timer Mode Selection Register)

TCON register

TCON is an 8-bit register, which controls the timer1 and timer0.

Bits and functions of every bits of TCON are as follows:

BIT	Function
TF1 / TF0	·      Timer 1 / Timer 0 overflow. ·      This bit is set (i.e. TF1=1 / TF0=1) by the microcontroller when timer1 / timer0 overflows.
TR1 / TR0	·      Timer 1 / Timer 0 run. ·      When this bit is set (i.e. TR1=1 / TR0=1) Timer1 / Timer0 is turn on. ·      When this bit is reset (i.e. TR1=0 / TR0=0) Timer1 / Timer0 is off.
IE1 / IE0	·      External interrupt (  / ) Edge flag. ·      This bit is set (i.e. IE1=1 / IE0=1) when external interrupt (  / ) arrives.
IT1 / IT0	·      External interrupt (  / ) triggered type control bit. ·      This bit is set (i.e. IT1=1 / IT0=1) when external interrupt (  / ) is edge triggered. ·      This bit is clear (i.e. IT1=0 / IT0=0) when external interrupt (  / ) is level triggered.

TMOD (Timer Mode) Register

Both Timer 0 and Timer 1 use the same register to set the various timer operation modes. It is an 8-bit register in which the lower 4 bits are set aside for Timer 0 and the upper four bits for Timer 1.

BIT	Function
Gate	·      Timer1 / Timer0 control. Every timer has a means of starting and stopping.  Timers do this by software or hardware. ·      This bit is reset (i.e. Gate=0) when timer 1 / timer 0 controlled by software. It means when this bit is reset, timer1 / timer0 will start when TR1 / TR0 bit of TCON is set. ·      This bit is set (i.e. Gate=1) when timer 1 / timer 0 controlled by hardware. It means when this bit is set, timer1 / timer0 will start after arriving of external interrupt (  / ) (provided that TR1 / TR0 bit of TCON should be set).
	·      Timer or Counter selector. ·      When this bit is reset (i.e.  =0) Timer1 / Timer0 will work as timer. ·      When this bit is set (i.e.  =1) Timer1 / Timer0 will work as counter.
M1 & M0	M1 M0 Mode Operating Mode 0 0 0 13-bit timer mode, 0 1 1 16-bit timer mode, 1 0 2 8-bit auto reload mode, 1 1 3 8-bit Spilt timer mode.

FMA Unit 3.5 Different Modes of Timers

Different Modes of Timers

Mode 0 (13-Bit Timer Mode)

·         Mode 0 is selected when M1 = 0 & M0 = 0 placed in TMOD SFR.

·         This is 13 bit timer can be loaded as TLx = 5 bits and THx = 8 bits

·   TLx can load value starting from 0 to 31 (001FH). When TLx is incremented above 31, it will reset to 0 and then start incrementing THx.

·         As this 13 bit timer, that counts starting from 0 to 8191. After that it will overflow.

·         When C/T(bar) , then timer will work as a timer to provide time delay.

·         When C/T(bar) , then timer counts external clock pulses applied at pin Tx.

  

Mode 1 (16-Bit Timer Mode)

·         Mode 1 is selected when M1 = 0 & M0 = 1 placed in TMOD SFR.

·         This is 16 bit timer can be loaded as TLx = 8 bits and THx = 8 bits
·         This mode is most commonly used and it provides maximum delay.

·      TLx can load value starting from 0 to 255. When TLx is incremented above 255, it will reset to 0 and then start incrementing THx.

·         As this 16 bit timer, that counts starting from 0 to 65535. After that it will overflow.

·         When C/T(bar), then timer will work as a timer to provide time delay.

·        When C/T(bar), then timer counts external clock pulses applied at pin Tx.

Mode 2 (8 Bit Auto Reload)

·         Mode 2 is selected when M1 = 1 & M0 = 0 placed in TMOD SFR.

·         It is 8 bit auto reloaded mode.

·      Initially 8 bit count loaded into THx which is automatically copied into TLx.

·         When Timer starts, clocks are received by TLx. TLx goes on incrementing up to its maximum value 255 and set the TFx.

· This mode is commonly used for deciding baud rate in serial communication.

  

Mode 3 (Split Timer Mode)

·         Mode 3 is selected when M1 = 1 & M0 = 3 placed in TMOD SFR.

·         When Timer 0 is placed in mode 3, it becomes two separate 8-bit timers.

·         Timer 0 is TL0 and Timer 1 is TH0. Both the timers count from 0 to 255 and in case of overflow, reset back to 0.

·         All the bits that are of Timer 1 will now be tied to TH0.

·         When Timer 0 is in split mode, the real Timer 1 (i.e. TH1 and TL1) can be set in modes 0, 1 or 2, but it cannot be started / stopped as the bits that do that are now linked to TH0.
·         The real timer 1 will be incremented with every machine cycle.